The vibration present in a rotorcraft cabin is due essentially to the rotation of the main rotor that provides propulsion and lift, and also to the flow of air along the fuselage of the rotorcraft.
U.S. Pat. No. 5,903,077 discloses an antivibration device using eccentric flyweights to generate a force seeking to reduce the vibration of a structure. It comprises two sets of flyweights each provided with a motor which drives a pair of eccentric flyweight rotors, i.e. rotors having centers of gravity that are not situated on their axes of rotation. The rotation of each rotor produces a rotary unbalance. Thus, the device generates a sinusoidal resultant force in the direction perpendicular to the plane containing the axes of rotation of the rotors in any one set.
By using two sets, it is possible to generate a force of adjustable resultant, of frequency equal to the speed of rotation of the rotors, expressed in revolutions per minute (rpm), and of amplitude associated with the angular phase difference between the two sets.
Nevertheless, in the event of one of the motors breaking down, the sets cease to rotate at the same speed, which means that the resultant force is no longer under control. Consequently, the comfort of the occupants of the structure is greatly degraded and can even become worse than in the absence of any antivibration treatment.
Document FR 2 852 648 discloses a device for compensating a vibratory force to which a body is subjected, the main object of the device is to remedy the above-mentioned drawback. That device has two identical sets of two rotors with respective eccentric flyweights, the sets being disposed symmetrically about an axis of symmetry and the axes of rotation of the rotors being mutually parallel and orthogonal to said axis of symmetry.
A single motor of axis disposed perpendicularly to said axis of symmetry sets the rotors into rotation by driving an endless link passing via pulleys mounted coaxially on the rotors so that the lengths of the strands of the link passing through said sets are equal. In addition, the motor is carried by controllable moving equipment capable of sliding along the said axis of symmetry in order to control the phase difference between the eccentric flyweight rotors of the sets.
Thus, a stable vibratory resultant force of determined amplitude and orientation can be generated by that device by moving the controllable moving equipment carrying the motor along the axis of symmetry, said movement acting via said link to vary the angular phase difference between the rotors of the sets progressively so as to bring the eccentric flyweights into the desired position.
In addition, a breakdown of the motor or a break of the endless link causes the device to come to a complete stop and therefore does not make the situation any worse.
Nevertheless, for the device to be accurate, the movement in translation of the controllable moving equipment carrying the motor along the axis of symmetry needs to be extensive. Unfortunately, since this movement in translation is directly linked to the diameter of the pulleys, the pulleys must be large in size. Consequently, it becomes difficult to arrange that device in a small space, such as a rotorcraft cabin, insofar as the space it occupies is not thoroughly optimized.